To: Olivia Dorothy March 9, 2018 Associate Director Upper Mississippi River Basin American Rivers

To: Olivia Dorothy March 9, 2018 Associate Director Upper Mississippi River Basin American Rivers From: Jonathan W.F. Remo Ph.D. Big Muddy Consulting Re: Assessment of flood surcharge related to differences in existing and authorized levee elevations along the Mississippi River between Keokuk, IA and Thebes, IL using the U.S. Army Corps of Engineers Upper Mississippi River Flood Risk Management Model This letter report documents the procedures and findings of a flood surcharge assessment using the U.S. Army Corps of Engineers (USACE) Upper Mississippi River Flood Risk Management Model (UMR FRM; USACE, 2018a). This assessment consisted of running the UMR FRM model with its two model geometries, one which contained the existing levee elevations, and the second that contained the federally-authorized levee design elevations along the Upper Mississippi River between Keokuk, IA and Thebes, IL. Four flood scenarios were evaluated to assess the impact the differing levee elevations had on water-surface elevations and maximum flood depths. A description of the UMR FRM model, surcharge assessment methods, results, discussion, and conclusions are reported below. Model Description: The UMR FRM model geometry was developed using the best available topographic and bathymetric data. Topography for the floodplain was developed using LiDAR based terrain data collected for the Upper Mississippi River Restoration Program Long-term Resource Monitoring element between 2008 and 2011(USACE, 2016). Holes within this data set were filled with state collected LiDAR terrain data. The bathymetric data consisted of a combination of single and multi-beam sonar surveys collected between 1999 and 2015. These topographic data were combined through interpolation to create a topobathy digital elevation model (DEM) with a 2-meter resolution. Levee elevations for levee systems which participate in the USACE s PL 84-99 levee safety program were compiled from the latest National Levee Database (NLD) surveys. Land cover from the U.S. Geological Survey s (USGS) National Landcover Database (USGS, 2011) were used to guide the selection of roughness coefficient applied in the hydraulic model. Information on bridges, navigation dams, inline structures, and river training structures were compiled for parameterization within the hydraulic model. River discharges and water-surface elevations used in the development of the UMR FRM model were compiled from USGS and USACE hydrologic monitoring stations, respectively. In addition, the National Weather Service s River Forecast models were used to estimate inflows from ungauged tributaries. All of these data were compiled using the tools within the USACE s Hydrologic Engineering Center s River Analysis (HEC-RAS v. 5.0.3) and its associated GIS tools, HEC-GeoRAS, to create a 1-D/2-D hydraulic model for the investigated river segments. The channel of the Mississippi River and its substantial tributaries were modeled as 1-D elements using the full momentum equation. Areas behind the main-stem and some of the major tributary levees were modeled as 2-D flow areas. Modeling the levee protected floodplain as 2-D flow areas allowed for a more realistic representation of flow into and through these portions of the floodplain. The 2-D element of the UMR FRM model uses the diffusion wave equation to 1

simulate flow within the levee protected floodplains of the Mississippi and its major tributaries. Discharge hydrographs were used for the upstream boundary condition at Keokuk, IA and for lateral inflow locations along the modeled river segments. Stage hydrographs or a rating curve were used for the downstream boundary condition at Thebes, IL (USACE, 2018b). Model scenarios and calibration: The UMR FRM model contains five flood scenarios. Four flood events, May-June 2008, April 2013, June-July 2014, and April-May2017, were the floods used to calibrate the UMR FRM model. Calibration results revealed a reasonably well performing model with root-mean-squared and mean-absolute error values for differences between predicted and observed water-surface elevations ranged from 0.01 up to 2.09 ft with an average error of ~1.0 ft. For all the flood scenarios included within the UMR FRM model, only levee breaches during the 2008 flood within the District were assessed. For all the other model scenarios, levees that are overtopped are assumed not to breach. Breach analysis was not performed for the other flood scenarios because the data to model the breaches was not available. A fifth flood, not used for model calibration, was the 1993 gaged inflow scenario. This flood scenario was included because modeling potential outcomes for such a large flood event is useful for evaluating impacts of flood-risk reduction efforts such as changes in levee height or configuration. However, this scenario is not a realistic recreation of the 1993 flood. This is because no effort was made to account for the impacts of flood fighting or levee breaching on water levels or discharges in order to make it a realistic recreation of the 1993 flood (USACE, 2018b). Authorized Levee Geometry: In addition to the existing condition geometry, an authorized levee elevation geometry was provided with the UMR FRM model. The authorized levee elevations were determined from as-constructed or design drawings from general design memorandums or the operation and maintenance manuals for each levees system. The authorized levee elevations are the net grade the levee system was intended to be after any post construction settlement or compaction. The net-levee grade elevations were edited to incorporate the approved Section 408 alteration requests for the District levees. Net levee grade elevations were then converted from the original datum to NAVD 1988. Authorized levee elevations are not available for non-federal levee systems and therefore the NLD information was used for non-federal levees in the authorized levee geometry. For non-federal levees for which NLD information was not available, the elevation from the Topobathy DEM were used to establish top of levee elevations in the authorized levee geometry file (USACE, 2018b). Assessment of Surcharge Between Existing and Authorized Levee Geometries: To assess for differences in water-surface elevations related to changes in levee heights, the 1993 gaged inflow, 2008, 2013, and 2017 flood scenarios were run in HEC-RAS v. 5.0.3 using both the existing and authorized levee geometries. The 2014 flood scenario was not run because the magnitude of this flood was insufficient to substantially inundate any of the levees systems along the modeled river segment. For the existing levee geometry, plans for the flood scenarios were provided and were run without any changes to the input parameters. To run the four flood scenarios with the authorized levee elevations, the associated geometry file was imported into HEC-RAS. However, during the HEC-RAS software s pre-check for input errors, eight issues were detected. These errors were associated with new connections in the authorized levee 2

elevation geometry. These connections were located between the 2D flow and storage areas in which a mesh face was overlapping between two model elements. To overcome these issues, the original model boundaries from the existing condition geometry were used instead of the new boundaries contained within the authorized levee geometry. Using the existing condition boundaries between the 2D flow and storage areas allowed for successful completion of the model runs using the authorized levee geometry. Comparison of the model boundaries between the existing and authorized levee geometries did not reveal any differences, except there was no overlap between the 2D flow and storage areas. This indicates using the original working 2D flow and storage area boundaries should not substantially affect the results for the authorizedlevee elevation model runs. Upon completion of the model runs, tabular discharge and water-surface elevation data were exported into Microsoft Excel to assess changes in these parameters between the existing and authorized levee height geometries for each of the assessed flood scenarios. Raster layers of the maximum water-surface elevations for the 1993 gaged inflow and 2008 flood scenarios were exported from HEC-RAS into ArcMap (v. 10.4.1) GIS software. The water-surface elevation raster layers for these flood scenarios for both the existing and authorized levee elevation model runs were subtracted from the topobathy DEM to calculate and map flood depths. The existing and authorized levee elevation flood-depth maps were subtracted from one another to calculate changes in flood depths. Results: Comparison of the UMR FRM model results using the existing and authorized levee elevation geometries for the 1993 inflow scenario revealed changes in maximum water-surface elevations from -0.38 up to 1.67 ft with an average increase of 0.52 ft. The most substantial increases in maximum water-surface elevation (>1.0 ft) occurred from La Grange to near Hannibal, MO. Other substantial increases in maximum water-surface elevations (>0.5 ft to 1.0 ft) were also noted between Keokuk, IA to La Grange, MO, Hannibal to Clarksville, MO, and Jefferson Barracks to Cape Girardeau, MO (Figure 1). The largest increases in the maximum water-surface elevations occurred within and just upstream of where the USACE (2017) levee surveys identified six levee systems which raised substantial portions of their levees two to four feet above the authorized design. Figure 2 shows the comparison of flood depths for the maximum water-surface elevations between the existing and authorized levee elevations for the 1993 gaged inflow scenario. The modeling results suggest under current levee conditions, the Sny Island Levees systems (reaches 1, 2, and 3) would not be substantially flooded during this scenario. However, at the authorized levee elevations the Sny Island Levees would be substantially flooded with depths exceeding 16.0 ft. in some areas. Fabius and South River levee systems, which also substantially raised their levees, saw large reductions in flood depths (6-12 ft). Elsewhere along the modeled river segment, substantial decreases in flood depths (5-16 ft) were noted within the Des Moines and Mississippi and the Harrisonville / Fort Chartres levee systems (Figures 2 and 3). For the 2008 flood scenario, comparison of the UMR FRM model runs using existing and authorized levee geometries revealed changes in maximum water-surface elevations ranged from 0.0 up to 1.5 ft with an average increase of 0.72 ft. The most substantial increases in maximum 3

water-surface elevations (>1.0 ft) generally occurred in the same locations as the 1993 gaged inflow scenario. Other substantial increases in maximum water-surface elevations (>0.5 to 1.0 ft) were noted between Gregory to La Grange, MO and from the Cuivre River to the Big Muddy River confluences (Figure 4). Comparison of flood depths for the maximum water-surface elevations between the existing and authorized levee heights also suggest under current levee elevations the Sny Island (reaches 1, 2, and 3) would not be substantially flooded. However, model results using the authorized levee elevations indicated Reach 1 of the Sny Island would be substantially inundated with flood depths >16.0 ft. in some areas. Five other levee districts, the Des Moines and Mississippi, Indian Grave North, Fabius, Marion County, and South River were substantially less flooded in the model run with existing levee system elevations (Figure 5). Discussion: Comparison of modeled water-surface elevations between the existing and authorized levee elevations for the 1993 gaged inflow and 2008 flood scenarios indicated nine levees systems have increased their levee heights. The modeling results also show the increased levee heights would reduce flood inundation and consequently flood risk within these systems. However, the modeling results also indicated raising these nine levee systems above the federally-authorized levee elevations increased water-surface elevations and consequently flood risk for unprotected floodplain areas and levees systems which have not increased their levee heights (Figures 2, 3, and 5). Levee surveys by USACE (2017) indicate the Fabius, Marion, Southern River and Sny Island Levee Districts have raised substantial portions of their levees by 2 to 4 ft. above the federally-authorized elevations. The modeling results presented here also suggest three other levee systems have raised their levees above the levee elevations contained within the UMR FRM model s federally-authorized levee geometry. These levee systems include the Des Moines and Mississippi, Harrisonville / Ft. Chartres, and Indian Grave North levee systems (Figures 2, 3, and 5). The analysis performed here could not determine if 14 of the levee systems which were not overtopped in the flood scenarios provided in the UMR FRM model were raised above the federally-authorized levee elevations. These 14 levee systems are located in the District and were not measured in the 2017 District s levee surveys (USACE, 2017; Appendix A). Hence, a comparison of levee heights between the existing and authorized levee elevations needs to be under taken to assess which, if any, of these 14 levees systems may also have been substantially raised. Differences in the pattern of inundation between the 1993 gaged inflow and 2008 flood scenarios are attributed to hydrologic differences between the floods, changes in levee system geometry, and methods in which levee system inundation were modeled (i.e., levee breaching in 2008 flood verses only levee overtopping in the 1993 gaged inflow scenarios). While the magnitudes of the 1993 gaged inflows and 2008 flood scenarios were similar above the Illinois River confluence, the duration of the 1993 flood was much longer than the 2008 flood (i.e., one month verses one week for discharges exceeding the100-year event). The longer duration of the 1993 flood generally allowed for more filling of the impacted levees systems. For example, under the 1993 gaged inflow scenario with the authorized levee elevations, three of the four Sny Levees were substantially inundated where under the 2008 flood scenario only the Reach 1 levee was substantially flooded (Figures 2 and 5). 4

Increases in water-surface elevations upstream and along reaches of river where levees have been raised are primarily attributed to hydraulic impacts of increase levee heights. Increases in water-surface elevations downstream of raised levees are chiefly attributed to increases in discharge. Raising the heights of levees can cause downstream increases in discharge by reducing the volume of flood waters that otherwise would have been stored within the levee system whose elevations have been substantially raised. The increase in discharge would consequently result in higher water-surface elevations downstream of the raised levees. The increase in river discharge is the reason why there are notable increases in maximum watersurface elevations below the Missouri River confluence to Thebes, IL for the 2008 flood scenario even though this was a minor flood (<20-year event) in which changes in levee heights along this river reach would have little to no impact on water-surface elevations (Figures 1 and 4). Summary: Comparison of water-surface elevations between UMR FRM model runs with existing and authorized levee elevations for the 1993 gaged inflow and 2008 flood scenarios indicate increases in levee heights along nine levee systems. The modeling results also suggest the increased levee heights have resulted in higher water-surface elevations for large floods (>50-year event) throughout a majority of the UMR modeled segment. While increased levee elevations have likely reduced flood risk for areas protected by these nine levee systems, they have increase flood heights and consequently flood risk for unprotected floodplain areas and for levee systems which have maintained the federally-authorized levee elevations. The 14 levee systems which were not overtopped by the flood scenarios modeled in this assessment were not evaluated to determine if they exceed the federally-authorized levee elevations. In addition, these 14 levee systems are located within the District and were not measured during the 2017 District s levee surveys (USACE, 2017; Appendix A). A comparison of levee elevations between the existing and authorized geometries needs to be undertaken to assess which, if any, of these 14 levee systems have been raised beyond the federally-authorized elevations. 5

Figure 1. Difference in maximum water-surface elevations for the 1993 gaged inflow scenario for model runs using existing and authorized levee geometries within the Upper Mississippi River Flood Risk Management Model 6

Figure 2. Comparison of maximum flood depths and difference in flood depths between the Upper Mississippi River Flood Risk Management Model existing and authorized levee elevations for the 1993 gaged inflow scenario between Warsaw, IL and Louisiana, MO. (A) maximum flood depth for the existing elevations; (B) maximum flood depth for the authorized design levee elevations; and (C) difference between maximum flood depth between existing and authorized design levee elevations. 7

Figure 3. Comparison of maximum flood depths and difference in flood depths between the Upper Mississippi River Flood Risk Management Model existing and authorized levee elevations for the 1993 gaged inflow scenario between Missouri River Confluence and Chester, IL. (A) maximum flood depth for the existing elevations; (B) maximum flood depth for the authorized design levee elevations; and (C) difference between maximum flood depth between existing and authorized design levee elevations. 8

Figure 4. Difference in maximum water-surface elevations for the 2008 flood scenario for model runs using existing and authorized levee geometries within the Upper Mississippi River Flood Risk Management Model 9

Figure 5. Comparison of maximum flood depths and difference in flood depths between the Upper Mississippi River Flood Risk Management Model existing and authorized levee elevations for the 2008 flood scenario between Warsaw, IL and Louisiana, MO. (A) maximum flood depth for the existing elevations; (B) maximum flood depth for the authorized design levee elevations; and (C) difference between maximum flood depth between existing and authorized design levee elevations. 10

References: U.S. Army Corps of Engineers (USACE). 2004. Upper Mississippi River flow frequency study. Available: http://www.mvr.usace.army.mil/missions/ Flood-Risk-Management/Upper-Mississippi-Flow- Frequency-Study/ U.S. Army Corps of Engineers (USACE), 2016. Upper Mississippi River Restoration (UMRR) Program Long Term Resource Monitoring (LTRM) element. 2016, UMRR Pool 19 through Open River South Topobathy: La Crosse, WI, https://www.umesc.usgs.gov/ U.S. Army Corps of Engineers (USACE), 2017. Letter report on the Upper Mississippi River Levee Elevations. District. May 10, 2017 p. 5 U.S. Army Corps of Engineers (USACE), 2018a. Upper Mississippi River Flood Risk Management Hydraulic Model. District, February, 2018. U.S. Army Corps of Engineers (USACE), 2018b. Upper Mississippi River Flood Risk Management Existing Conditions Hydraulic Model Documentation Report. District, February, 2018. p. 30. U.S. Geological Survey (USGS), 2014. National Land Cover Dataset 2011 Land Cover National Geospatial Data Asset (NGDA) Land Use Land Cover, U.S. Geological Survey, Sioux Falls, SD. 11

Appendix A - Levee height assessment for levees between Keokuk, IA and Thebes, IL U.S. Army Corps of Engineers District Name Levee Height Assessment Levee Height Assessment Source Canton LFPP Within 2 ft. of authorized elevation USACE, 2017 USACE (2017) levee survey indicates levee elevations are within 2 ft. of their authorized Des Moines and Elevation. However, modeling results suggest USACE, 2017; Mississippi Levee current levee is higher in elevation than above District the Upper Mississippi River Flood Risk Management Model's Authorized Levee Geometry Fabius River LFP Portions exceeding 2 ft. of authorized elevation USACE, 2017 Gregory D&LD Within 2 ft. of authorized elevation USACE, 2017 Hunt-Lima DD Within 2 ft. of authorized elevation USACE, 2017 USACE (2017) levee survey indicates levee elevations are within 2 ft. of their authorized Indian Grave Elevation. However, modeling results suggest USACE, 2017; Drainage District - current levee is higher in elevation than above South the Upper Mississippi River Flood Risk Management Model's Authorized Levee Geometry Indian Grave Drainage District - North Within 2 ft. of authorized elevation USACE, 2017 Marion County DD Portions exceeding 2 ft. of authorized elevation USACE, 2017 Mississippi - Fox Drainage District Lower Sny Island Levee Drainage District - Reach 1 Sny Island Levee Drainage District - Reach 2 Sny Island Levee Drainage District - Reach 3 Sny Island Levee Drainage District - Reach 4 Within 2 ft. of authorized elevation USACE, 2017 Portions exceeding 2 ft. of authorized elevation USACE, 2017 Portions exceeding 2 ft. of authorized elevation USACE, 2017 Portions exceeding 2 ft. of authorized elevation USACE, 2017 Within 2 ft. of authorized elevation USACE, 2017 South Quincy DD Within 2 ft. of authorized elevation USACE, 2017 South River Drainage District Portions exceeding 2 ft. of authorized elevation USACE, 2017 12

U.S. Army Corps of Engineers District Name Union Township LFP Levee Height Assessment Levee Height Assessment Source Within 2 ft. of authorized elevation USACE, 2017 Big 5 Levee Bois Brule L&DD Brevator Levee Chouteau Island / Chain of Rocks West Levee Columbia Bottoms Columbia D&LD No.3 Consolidated North County Levee Elm Point Levee Elsberry/King's Lake Foley, Cap Au Gris Levee & Winfield Main Grand Tower / Degonia Harrisonville / Ft Chartres Modeling results suggest current levee is higher in elevation than above the Upper Mississippi River Flood Risk Management Model's Authorized Levee Geometry 13

U.S. Army Corps of Engineers District Name Levee Height Assessment Levee Height Assessment Source Kaskaskia Kissinger Levee Kuhs Levee Lakeside 370 Metro East/ Chain of Rocks Pike Grain Levee No.4 Prairie Du Pont & Fish Lake Prairie Du Rocher Sandy Creek Levee St Louis Flood Protection Project St. Genevieve No. 2 St. Genevieve No. 3 St. Peters Missouri Old Town Levee Stone Murdoch 14

U.S. Army Corps of Engineers District Name Levee Height Assessment Levee Height Assessment Source Winfield Pin Oaks Wood River D&LD Lower Wood River D&LD Upper 15